The present invention relates to a soldering method for soldering lead wires on wiring patterns of a printed circuit board after inserting the lead wires of an electronic device that protrudes downward via through holes of the printed circuit board having the wiring patterns provided on the rear side thereof.
A so-called dip process as shown in FIGS. 4 and 5, has been employed as such a soldering method, for example.
In the process, lead wires 1a of an electronic device 1, which protrude downward, is first inserted into through holes 2b of a printed circuit board 2 on which wiring patterns 2a are provided, and are immersed in a soldering tank 3 in which solder is heated and melted at above the melting temperature, and lifted up therefrom, wherein solder provided in the soldering tank 3 is adhered to the lead wires 1a and is hardened in a required time, and the lead wires la are soldered to the wiring patterns 2a of the printed circuit board 2, thereby forming soldered parts 4.
Also, although not illustrated, a so-called reflow process has been employed, in which cream soldering is coated in advance on the soldering portion on the wiring patterns on the rear side, after the lead wires are inserted through the through holes of a printed circuit board, and hot air is partially applied to the soldering portion on which the cream solder is provided, or causing the printed circuit board to pass through a high temperature room whose temperature is higher than the melting temperature of the cream solder, whereby the cream solder is melted, thereby performing soldering.
However, such a process has various types of problems as described below:
That is, in the case of the dip process, as lead wires are immersed in the soldering tank 3 and lifted up therefrom, the solder is raised along the lead wires, and at the same time the solder is spread on the wiring patterns 2a and 2a secured on the rear side of a printed circuit board 2 by the surface tension, wherein the adjacent wiring patterns 2a are brought into contact with each other, wherein there is probability that an electric short-circuit may occur.
Also, in order to prevent such a situation from occurring, the amount of adhered solder 4 becomes short, wherein there is another problem in that the electrical properties may be worsened.
Taking the above problems into consideration, it was very difficult to control and adjust the soldering work in satisfactory conditions.
In the case of the reflow process, lead wires are inserted into through holes of a printed circuit board and cream solder is coated in advance onto soldering portions on the wiring patterns on the rear side thereof, wherein hot air is directly applied thereto to melt the solder for soldering. However, since hot air is blown thereon, the solder may be oxidized, thereby worsening the characteristics of the solder. Also, where hot air is directly blown thereon, the temperature of the printed circuit board is raised before the cream solder reaches its melting temperature, electronic devices secured on the printed circuit board are accordingly heated, whereby the characteristics of the electronic devices may be worsened.
Further, in the case of a process of passing a printed circuit board through a high temperature room whose temperature is retained at the melting temperature of the solder or above the characteristics of other electronic devices mounted on the printed circuit board other than those to be soldered are worsened, and electronic devices that are adversely affected by heat cannot be mounted.
The present invention was developed in view of the above-described problems and shortcomings. It is therefore an object of the invention to provide a soldering method, by which desired electrical characteristics can be secured, no soldering defect is brought about due to oxidization, the influence on electronic devices mounted on the surface of a printed circuit board can be suppressed, and resultantly electronic devices that are not resistant against heat may be mounted.
In order to achieve the above object, according to the invention, there is provided a method of soldering a lead wire of an electronic device to a wiring pattern formed on a printed circuit board, comprising the steps of:
placing the electronic device on a first face of the printed circuit board such that the lead wire thereof are protruded from a second face of the printed circuit board, on which the wiring pattern is formed;
coating cream solder on the protruded lead wire;
providing a liquefied heat medium having a temperature higher than a melting temperature of the cream solder;
immersing the second face of the printed board in the liquefied heat medium in a predetermined time period, to melt the cream solder; and
lifting the printed board from the liquefied heat medium.
According to the above method, after the lifting the printed board, the melted cream solder coated on the lead wires is hardened to solder the lead wires. Therefore, satisfactory soldering can be obtained.
In this method, since an own weight of the melted cream solder is counterbalanced by a diffusion force resulting from a surface tension on the wiring pattern, the undesired contact of the adjacent wiring patterns due to the diffusion of the melted solder as in the dip process will not occur.
Further, since the heat medium to melt the solder is liquefied, no oxidization is allowed to occur, wherein it is possible to prevent the characteristics of the solder from worsening in comparison with the reflow process.
Still further, since the influence of heat can be minimized at not only the electronic devices to be soldered but also other electronic devices mounted in the printed circuit board. In particular, electronic devices that are likely to be affected by heat can be mounted. Accordingly, it will become possible to incorporate remarkably multi-purposed electronic devices.